There is no way we should go into the season with Umtiti/Lenglet as backup LBs. Alba is injury prone and then we have Vermaelen (if he stays) as one of the 4 CBs who we can also expect to miss at least a month or two if not more every season because of injuries so Umtiti and Lenglet will both be needed to play in their primary position.
I like Cucu and hope he gets a chance in preseason but he's unproven beyond Segunda A level and we shouldn't keep him to play in "easy games" only. If Alba gets injured/suspended before El Clasico or CL quarter final and we can't count on Cucu to play in those kind of games then it's better to keep Digne. He's as good backup as we can get and I wouldn't sell him for only 17M unless we can get someone better for not much higher price, preferably a versatile player who could also play RB, CB and/or another position. But those kind of players are rare.
So personally I would keep Digne, loan/sell (with buy back) Cucu, let Miranda develop in Barça B while occasionally train and possibly also get some games with the first team and then re-evaluate things next summer when we'll also have to decide what to do with Alba if he won't renew before. Which I think he will but don't know if this will really be a good decision longterm.